This invention relates to improved systems for preventing contamination of dry gas seals, which require a continuing or non-interrupted flow of clean seal gas into and across the relatively moving faces of the dry gas seal.
Dry gas seals are used in various applications such as centrifugal compressors and pumps as well as axial flow compressors, and especially compressors which operate at high pressures, such as compressors used in gas transmission, process or utility industries for boosting gas pressure.
In most types of compressors commonly used for boosting pressure in gas transmission, process or utility industries, one or more centrifugal or axial flow impellers are mounted on a shaft and form a rotor which rotates within a gas space in the compressor housing thereby to move gas from a suction inlet to a discharge outlet of the space. The shaft may be of the beam type wherein the impeller or impellers are mounted between two radial bearings. Alternatively, the shaft may be of the overhung type wherein the impeller or impellers are cantilevered from the two radial bearings. Any such compressor is usually coupled to a gas or steam turbine or an electric motor which provides the rotational drive power.
In such compressors, all of the space in which the impellers operate is pressurized to at least the pressure of the gas to be boosted, which in turn is dependent upon the process but which can be anywhere from several hundred to several tens of thousands of kPa. Leakage of the gas into the bearing cavity is controlled by seals. Dry gas seals are becoming the standard of choice in an increasing number of applications over older technology such as oil film seals. In dry gas seals, the sealing function is provided by a very thin film of gas which is permitted to pass between two relatively rotating annular surfaces. The leakage across the faces of such dry gas seals is quite low even when the pressure differentials are quite high.
Essential to effective, efficient and reliable operation of such dry gas seals is the provision of a steady supply of clean gas, free of contamination which could otherwise migrate between the relatively rotating annular seal surfaces and damage the surface of one or both of them due to abrasion. Such abrasion can cause a disruption in the ability of the relatively rotating annular faces to form a stable sealing film, or restrict the range of operating conditions over which such a sealing film may be reliably established, or otherwise degrade the measurable performance of the seal. Therefore it is desirable to prevent such contamination from migrating into the space where the dry gas seals are located.
A seal supply gas system is therefore provided in an effort to avoid the contamination problem, which system may or may not make use of the compressor process gas as the source. Regardless of the gas source, it is filtered and the volumetric flow regulated by some means, so as to provide the seal cavity with a steady supply of clean gas, at an optimum predetermined flow rate, conducive to reliable dry gas seal operation.
In cases where the source of the seal supply gas is the compressor process gas, the source is supplied from the discharge side of the compressor. During normal operation, this gas will have a higher pressure than the pressure in the seal cavities of both beam type and overhung type compressors. The potential then exists to drive the gas through the seal supply gas system, consisting of piping, connections, filters, valves and flow rate regulator. The gas will flow through this system and provide the seal cavity with a volume of clean gas such that the majority will flow through the seal cavity, across a labyrinth seal which separates the seal and process cavities, and thence back into the process cavity where it is re-compressed. Only a fraction of this volume will flow across the relatively rotating annular faces of the dry gas seals, and be vented, disposed of, or otherwise recaptured outside of the compressor casing.
During gas compressor operation in modes where a low pressure differential exists between the suction and discharge sides of the gas compressor, the aforementioned potential to drive the seal supply gas from the discharge side of the gas compressor through the seal supply gas system will not exist. This can occur during the startup sequence of the compressor. In such modes of operation, the risk of process gas flowing backwards from the process cavity into the seal cavity is greatly increased. If this occurs, the risk of process contamination migrating into the seal cavity and between the relatively rotating annular faces of the dry gas seals is greatly increased. Contamination entering seal cavities during this operating mode has been known to be sufficient to damage the seal to such a degree as to render it inoperable.
The prior art provides various systems which attempt to introduce a continuous flow of pressurized seal gas into enclosed seal cavities for the purpose of keeping them free from contamination. However, none of them provide for a backup system to maintain the flow of seal gas in the event that the primary source of such pressurized seal gas becomes inoperable for whatever reason.
An object of the invention is to provide a system for preventing process contamination from entering the dry gas seal cavity, migrating into the moving parts of the dry gas seal itself, and causing degradation in seal performance, or failure thereof in the course of certain varying conditions, e.g. compressor output pressure fluctuations, which may be encountered during operation.
In accordance with the invention in one aspect there is provided in combination, a turbomachine having a rotor therein, the rotor having a dry gas seal associated therewith, and a primary seal gas supply system adapted to supply a flow of seal gas to the dry gas seal when a sufficient pressure differential exists as to maintain that flow, the primary seal gas supply system having an upstream section adapted to be connected to a primary source of the gas and a downstream section connected to the dry gas seal, a one-way valve between the upstream and downstream sections preventing a back flow of gas from the downstream section into the upstream section; and a seal gas pressure booster system including a booster compressor having a gas inlet line connected to the upstream section of the primary seal gas supply system and a gas outlet line connected to the downstream section of the primary seal gas supply system for boosting the pressure of the gas using said primary seal gas supply in the downstream section sufficiently to ensure a continuing flow of seal gas to the dry gas seal during periods when there is an insufficient seal gas pressure differential associated with the primary system as to maintain the desired flow of seal gas therein, means for producing a signal indicative of insufficient seal gas pressure differential, and means associated with the booster compressor for driving the latter to boost the gas pressure in the downstream section in response to the signal.
In accordance with the invention in a further aspect there is provided a method of boosting seal gas pressure in a seal gas supply system for a turbomachine having a rotor therein equipped with a dry gas seal, the seal gas supplying system having an upstream section connected to a primary source of the gas and a downstream section connected to the dry gas seal, a one-way valve between the upstream and downstream sections preventing a back flow of gas from the downstream section into the upstream section; the seal gas pressure boosting method including providing a booster compressor having a gas inlet line to the upstream section and a gas outlet line connected to the downstream section sensing differential pressures between an inlet and an outlet of the turbomachine and producing a signal in response to the presence of a pre-selected differential pressure, and driving the booster compressor in response to the signal which is indicative of insufficient seal gas pressure differential associated with the primary seal gas supply system as to maintain adequate flow therein thereby to boost the pressure of the gas using said primary source of said gas in the downstream section sufficiently to ensure a continuing flow of seal gas to the dry gas seal.
In accordance with one embodiment, the invention is applied to a gas compressor having dry gas seals and a seal supply gas system wherein the source of seal supply gas is also the compressor discharge process gas, (which exists from time to time during operation at a pressure insufficient to overcome the flow losses associated with the seal supply gas system). The pressure booster system according to the invention detects this insufficient pressure by effecting a comparison with a predetermined setpoint, and upon detection, a valve opens to admit the gas normally used for the seal supply into a positive displacement booster compressor. This booster compressor raises the pressure of the seal supply gas to a level sufficient to overcome the flow losses associated with the seal supply gas system. The booster compressor continues operation until the operating mode of the gas compressor changes such as to provide a discharge pressure sufficient to provide the necessary potential to overcome the losses associated with the seal supply gas system. At this point, the positive displacement compressor shuts down, and seal supply gas is supplied as if the booster system were not present.
The seal gas pressure booster system is therefore particularly useful where the process gas in question contains contamination which has the potential to cause damage to the relatively rotating annular faces of the dry gas seal. Although dry gas seals come in a variety of configurations, which may include multiple stages, the booster system according to the invention is always situated to promote buffering of the first sealing stage of the dry gas seal from the process cavity.